The present invention relates generally to a vessel at least a bottom wall of which is adapted to be heated through high-frequency electromagnetic induction heating for heating fluid contents of the vessel. More particularly, the invention is concerned with pots and pans usable on a high-frequency electromagnetic induction cooking heater.
A high-frequency induction heating instrument such as a cooking apparatus using such an induction heater, is provided with a top plate on which is placed a suitable vessel such as a pot or pan, and further provided with an inductor coil disposed on the lower side of the top plate. The bottom plate or bottom wall of the vessel on the top plate is heated with heat produced by high-frequency eddy currents which are induced in the bottom wall of the vessel by the electromagnetic field produced by the inductor coil of the heater. Consequently, the materials for the pots and pans for an induction-heating cooking apparatus are limited to magnetic metals such as iron and stainless steel. However, as the magnetic metals are relatively low in coefficient of thermal conductivity, pots or pans made of such magnetic metals suffer an inconvenience of uneven heat distribution through a mass of the contents to be cooked, particularly in the case where the contents to be cooked are materials for curry sauce (curry paste), stew or the like which are not expected to undergo a sufficient degree of convection, where the pot or pan has a comparatively large depth, and where an induction-heating cooking apparatus used is designed to heat only the bottom portion (bottom wall) of the pot or pan. That is, the temperature of the side wall of the vessel is held relatively low due to low thermal conductivity of the magnetic metal of the vessel and due to poor convection of the contents to be heated, thereby making it difficult to achieve even or uniform heating of the contents of the vessel, which is an essential requirement in cooking fluid materials in a pot or pan.
For cooking on a high-frequency electromagnetic heater, there has been known a pot which is a vessel made of aluminum or aluminum alloy having a high thermal conductivity. The bottom plate of the vessel is backed at its lower surface by an iron sheet (magnetic material). Although the aluminum or aluminum alloy body is highly thermally conductive, it permits a high degree of heat dissipation during heating of the vessel. In other words, a large amount of heat is dissipated in the atmosphere through the side wall of the vessel during cooking of the contents, thereby causing a heat loss which leads to difficulty to attaining even heating of the contents of the vessel. Also known is a pot which is press-formed from a composite material which is an aluminum sheet sandwiched on opposite sides thereof by stainless steel sheets. Although this arrangement permits good heat transfer towards an upper part of the side wall of the pot through the highly conductive core layer of aluminum, the heat in the side wall is blocked by the inner stainless steel layer with a low thermal conductivity, and thereby hindered from being transferred to the contents of the pot. Therefore, this type of pot also suffers the same inconvenience as indicated above.
To minimize the above inconvenience experienced in the prior art, the assignees of the present invention have proposed a vessel for a high-frequency induction cooking heater, as indicated in Japanese Utility Model Application No. 578-83036 (published on Dec. 9, 1983 under Publication No. 58-185149), wherein the outer side of the vessel is formed of iron, stainless steel or other magnetic material, which allows high-frequency induction heating of the vessel while preventing heat dissipation through the side wall outwardly of the vessel. On the other hand, the inner side of the vessel is formed of aluminum or other highly thermally conductive material. so as to improve the heat conduction towards the upper part of the side wall and consequently to the contents of the vessel.
At least a bottom portion or bottom plate of a vessel such as pots and pans as described above, for use with a high-frequency electromagnetic induction heater, is press-formed of a combination of a magnetic material and a highly thermally conductive metal. For example, the bottom portion is formed from a cladding consisting of a stainless steel sheet and an aluminum alloy sheet which are clad with each other under pressure, so that the aluminum alloy sheet is disposed on the inner side of the vessel. However, it has been found that such a bottom portion of the vessel has a drawback to be solved for improvement in the quality of the vessel. Stated more specifically, a vessel for use with an electromagnetic induction heater generally employs, for easier induction heating, a ferrite-based stainless steel. Such vessels for general domestic applications are made, for example, of a stainless steel material which consists of: 0.015% of carbon (C); 0.49% of silicon (Si); 0.54% of manganese (Mn); 0.026% of phosphorus (P); 0.003% of sulfur (S); 16.75% of chromium (Cr); 0.38% of copper (Cu); 0.52% of niobium (Nb); and the balance being iron (Fe). On the other hand, aluminum pots are commonly made of Al-Mn alloy as per AA Standard 3003. To make a blank material for a pot, a composite sheet, i.e., a cladding (clad metal sheet) was prepared by cladding these commonly used materials, Al-Mn alloy of AA 3003 and the stainless steel of the above indicated composition. The cladding was press-formed into shape such that the aluminum alloy layer was disposed on the inner side of the pot. This experimental manufacture revealed that the aluminum alloy layer (AA 3003) of the formed pot had a surface quality too poor to enable the pot to serve.